1. Field of the Invention
This invention relates to a power semiconductor device and particularly to a power semiconductor device having a super-junction structure.
2. Background Art
The ON resistance of a vertical power MOSFET (metal-oxide-semiconductor field effect transistor) greatly depends on the electric resistance of its conduction layer (drift layer). The dopant concentration that determines the electric resistance of the drift layer cannot exceed a maximum limit, which depends on the breakdown voltage of a p-n junction between the base layer and the drift layer. Thus there is a tradeoff between the device breakdown voltage and the ON resistance. Improving this tradeoff is important for low power consumption devices. This tradeoff has a limit determined by the device material. Overcoming this limit is the way to realizing devices with low ON resistance beyond existing power devices.
As an example MOSFET to solve this problem, a structure with p-pillar layers and n-pillar layers buried in the drift layer is known as a super-junction structure (see, for example, JP-A 2001-298191 (Kokai)). In the super-junction (hereinafter, also referred to as SJ structure), a non-doped layer is artificially produced by equalizing the amount of charge (amount of impurities) contained in the p-pillar layer with that contained in the n-pillar layer. Thus, while holding a high breakdown voltage, a current is passed through highly doped n-pillar layer. Hence a low ON resistance beyond the material limit is realized.
Thus the SJ structure can be used to realize a balance between ON resistance and breakdown voltage. However, when more reduction of the ON resistance and more increase of the breakdown voltage of the power semiconductor device using the SJ structure are attempted, the following problems are caused. That is, for more reducing the ON resistance, the impurity amounts of the p-pillar layer and the n-pillar layer can be increased. However, in this case, for fully depleting the p-pillar layer and the n-pillar layer, larger horizontal electric field is required, and therefore, vertical electric field determining the breakdown voltage is lower. As a result, the breakdown voltage becomes low. Moreover, for more increasing the breakdown voltage, the impurity amounts of the p-pillar layer and the n-pillar layer can be reduced to make it easy to deplete the pillar layers. However, in this case, the ON resistance increases. Furthermore, for reducing the ON resistance with holding a high breakdown voltage, an array period of the p-pillar layer and n-pillar layer and additionally the impurity amounts of the p-pillar layer and the n-pillar layer can be increased. However, in this case, process steps get complex, and difficulty of the process turns up.